A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium mounted on various magnetic recording apparatuses, and to a manufacturing method therefor.
B. Description of the Related Art
A perpendicular magnetic recording system in which recording magnetization is perpendicular to a medium surface is being explored as a technology for achieving higher density in magnetic recording than that achieved by conventional longitudinal magnetic recording systems.
A perpendicular magnetic recording medium is mainly composed of a magnetic recording layer of hard magnetic material, an intermediate layer for aligning the magnetic recording layer to a specified orientation, a protective layer for protecting a surface of the magnetic recording layer, and a backlining layer of soft magnetic material. The backlining layer concentrates magnetic flux generated by a magnetic head for recording on the recording layer. The soft magnetic backlining layer can enhance performance of a medium, but can be omitted since recording is possible without a backlining layer.
To achieve high recording density in a perpendicular recording medium, low noise compatible with high thermal stability is required as in a longitudinal recording medium.
In the field of longitudinal magnetic recording media, various compositions and structures of the magnetic recording layer and materials for the nonmagnetic intermediate layer have been proposed to date. Practically used magnetic recording layers employ an alloy composed mainly of cobalt and chromium (hereinafter referred to as a CoCr alloy). The chromium segregates to a grain boundary region and becomes a principal component of the grain boundary region to obtain magnetically isolated magnetic grains. Another magnetic recording layer is proposed in which the principal component of the grain boundary region is oxide or nitride. (See U.S. Pat. No. 5,679,473 and Japanese Unexamined Patent Application Publication No. 2001-101651.)
The above-mentioned magnetic materials in longitudinal magnetic recording media can also be applied to perpendicular magnetic recording media by, for example, providing an intermediate layer that controls crystal orientation to achieve perpendicular magnetic anisotropy. Reduction of magnetic interaction between magnetic grains also is a problem in attaining high recording density in perpendicular magnetic recording media.
An example using a CoCr alloy is disclosed in Japanese Unexamined Patent Application Publication No. 2002-358615. The magnetic recording layer in the reference uses CoCrPt—X, and the chromium concentration is higher in the grain boundary region than in the crystalline grains, obtaining a segregation structure.
Examples using oxide or nitride as the principal component in the grain boundary region are disclosed in Japanese Unexamined Patent Application Publication Nos. 2000-306228 and 2000-311329.
A segregation structure is obtained by heat treatment in the references. Drastic reduction of media noise in this type of recording layer, in comparison with a magnetic recording layer that uses a conventional CoCr alloy with a principal component of chromium in the grain boundary region, is disclosed in a reference: T. Oikawa et al., IEEE Trans. Magn., September 2002, vol. 38, No. 5, pp. 1976-1978, “Microstructure and Magnetic Properties of CoPtCr—SiO2 Perpendicular Recording Media”.
Perpendicular magnetic recording media recently have been proposed in which a magnetic recording layer is constructed with a multiple of function-separated layers to achieve not only a reduction in media noise, but also a high thermal stability. Japanese Unexamined Patent Application Publication No. 2002-230735 discloses a multi-layered structure of CoCrPt alloy layers containing chromium and platinum with concentrations that differ from layer to layer. Japanese Unexamined Patent Application Publication No. 2003-077133 discloses a multi-layered structure including a granular magnetic layer and an amorphous magnetic layer without a grain boundary. Here, the granular structure is a material structure in which ferromagnetic crystalline grains are surrounded by a nonmagnetic grain boundary region principally composed of oxide and/or nitride.
In magnetic recording devices equipped with a flying magnetic head, a distance between the magnetic head and the magnetic recording medium is a very small value of several tens of nanometers. Durability of this type of device is strongly affected by friction and wear characteristics between the head and the medium. To improve the friction and wear characteristics of the medium, the medium surface generally is coated with a liquid lubricant having a molecular weight of several thousands. It is known that precipitation of cobalt atoms contained in the magnetic layer of the medium to the medium surface promotes decomposition of the liquid lubricant on the medium surface, resulting in a substantial degradation of durability of the medium. To avoid the precipitation of cobalt, it is necessary to control thickness and quality of a protective film and surface roughness of a medium surface.
Studies by the inventers of the present invention, however, revealed that cobalt atoms contained in the magnetic layer readily precipitate to the medium surface in a magnetic layer having a granular structure. A magnetic layer having a granular structure is referred to as a granular magnetic layer. The cobalt precipitation is significant particularly when argon gas pressure in the sputtering deposition process is increased for achieving excellent magnetic properties and electromagnetic conversion performance. If the cobalt atoms precipitate to the medium surface, the cobalt atoms promote decomposition of the liquid lubricant molecules on the medium surface, creating a problem of substantial degradation of durability against friction and wear of the medium.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.